Nitride semiconductor devices featuring high saturated electron velocities and wide band gaps have been actively developed as semiconductor devices of high voltage resistance and high output. A large number of reports have been made regarding nitride semiconductor devices such as field effect transistors, in particular, high electron mobility transistors (HEMTs). AlGaN/GaN HEMTs that use GaN in the electron transit layer and AlGaN in the electron donating layer are drawing much attention. According to AlGaN/GaN HEMT, strain attributable to the difference in lattice constant between GaN and AlGaN occurs in AlGaN. Piezoelectric polarization caused by the strain and spontaneous polarization of AlGaN generate high-concentration two-dimensional electron gas (2DEG). Thus, high voltage resistance and high output are realized.
Japanese Examined Patent Application Publication No. 7-093428 and Japanese Laid-open Patent Publication No. 2008-112868 are examples of related art.